This invention relates to a process for the melt-spinning of a filamentary structure from a synthetic polyamide polymer. More particularly, it is concerned with an improved process for the formation of an improved antistatic filament, yarn or the like by melt-spinning a synthetic linear fiber-forming polyamide.
It has been suggested that the utility of synthetic fibers could be increased and their properties, in particular their antistatic properties, could be improved if a polyalkylene ether of high molecular weight is included in the polymer. More specifically, it is disclosed in U.S. Pat. No. 3,475,898 to Magat and Sharkey to use poly(ethylene-propylene)ether glycols for this purpose. More recently, U.S. Pat. No. 3,657,386 discloses that certain propylene oxide-ethylene oxide copolymers based on ethylene diamine are useful in preparation of an antistatic fiber of polyamide. It has also been suggested that the utility of synthetic fiber of polyamide could be increased by dispersing in the polyamide an antistatic compound which is a reaction product of: ##STR2## where a, b, c, d, w, x, y and z are each a whole number and R is a difunctional radical from a hydrocarbon containing 1 to 13 carbon atoms, said tetrol compound having a molecular weight between about 4,000 and about 50,000 and at least one compound selected from the group consisting of diepoxides and compound which yield the following divalent radicals: ##STR3## where R' is a difunctional radical from a hydrocarbon containing 1 to 30 carbon atoms. However, serious problems were encountered in melt-spinning due to the frequent occurrence of "nubs" in the fiber. The term "nubs" is conventionally applied and is used herein to mean enlarged sections of filament no more than several filament diameters in length. Nubs may be formed by a foreign, non-orientable substance which interferes with normal fiber stretch in a short section, resulting in an enlargement. Foreign substances which are believed to have contributed to nubs in the present instance include carbonized polymer from face of extruder die and spinnerette, and gels formed in the polymer. Gels appear to be the chief cause, i.e., the nubs are probably created by non-orientable gel from cross-linked polymer. Thermal degradation of the polymer may be an important causative factor.
The reactions in thermal degradation of polyamides containing polyalkylene ether additives are not entirely understood. It is likely that thermal degradation produces a decomposition product which serves to form cross-links between amide groups and adjacent polymer chains. The decomposition reaction proceeds slowly, finally building up a three-dimensional network of molecules which may be called polymer gel and which eventually reaches the stage where it forms an infusible coating on the walls of the reactor and other equipment.
A serious difficulty which arises from the formation of this polymer gel on the interior walls is that from time to time pieces break off and get into the flowing polymer stream where they produce damage to the spinning equipment.
The greatest difficulty, however, is caused by polymer gel which has progressed to the three-dimensional structural stage, but which has not yet reached the stage of being infusible. This kind of polymer gel is readily carried with the stream of flowing polymer. Being still molten or at least softened, it passes through the pump and even through the filter medium to show up either as discontinuities or as viscosity differences in the spun filament. When these filaments are later cold drawn, these defects may cause breaks in the filaments which either cause the whole thread to break or else form nubs which go through to be counted as quality defects in the final yarns.
More recently, it has been discovered that certain hindered phenol compounds may be incorporated into the antistatic polyamide to reduce the formation of nubs in the fibers. For example, U.S. Pat. No. 3,876,725, issued Apr. 8, 1975, to Rodney Lee Wells et al., discloses that the occurrence of said nubs in the antistatic polyamide fiber can be greatly reduced by dispersing in the polymer about 0.5 to 12 percent by weight, based on the weight of the antistatic agent, of a phenol compound represented by the formula: ##STR4## where R is an alkyl hydrocarbon group containing less than nine carbon atoms and R.sub.1 is a tertiary alkyl group containing at least four but not more than 12 carbon atoms. In comparative tests with antistatic yarns containing various commercially available hindered phenol compounds, the yarns of the patent showed significantly greater breaking strength retention after exposure to light in standard tests.
Although the antistatic polyamide fiber of U.S. Pat. No. 3,876,725 is an important contribution to this art, research in this field has been continued in an effort to produce an antistatic polyamide fiber having still greater breaking strength retention than the fiber of said patent. Moreover, the research was widened to amides containing hindered phenol groups in an effort to lower the volatility of the phenol compound to prevent vaporization losses during melt spinning of the polymer.